Mobile communication equipment represented by a cellular phone is rapidly advancing toward downsizing and extended talk time in recent years. For this reason, there is a strong demand for increasingly high efficiency in a transmission power amplifier which consumes a large portion of power during a call.
Especially, a recent communication system represented by CDMA (Code Division Multiple Access) has a so-called “power control function.” This is the function that a terminal communicates with a base station with reduced transmission output when the terminal is located in a short distance from the base station.
At this time, the operation of the transmission power amplifier is switched from high output power (approximately 27.0 dBm) to low output power (approximately 13 dBm). During this low output power period, the transmission power amplifier operates within a range where sufficient linearity is obtained. This allows a bias point to be reduced (reducing an operating current) while maintaining linearity. Thus, there is a mobile communication terminal configured to achieve high efficiency as shown in FIG. 9.
In FIG. 9, reference numeral 1 denotes a bias supply transistor, 2 denotes a radio frequency power amplifier transistor, 11, 12, 13, 16 denote resistors, 14, 15 denote Schottky barrier diodes for temperature compensation, 31 denotes a Vctrl voltage which determines a base potential of the bias supply transistor 1 and 32 denotes a Vcc voltage which gives a potential to the collector of the power amplifier transistor 2. An idle current value of the collector of the power amplifier transistor 2 is determined by a base current generated by the Vctrl voltage 31, bias supply transistor 1, resistors 11, 12, 13, 16, Schottky barrier diodes 14, 15, and makes the Vctrl voltage 31 variable and controls a bias point of the RF power amplifier transistor 2.
FIG. 10 shows a circuit described in Japanese Patent Laid-Open No. 2003-51720, which newly adds a power control transistor 3 and resistors 21, 22 to the emitter of the bias supply transistor 1 and applies a power control voltage 33 to the base of the power control transistor 3 through the resistor 21 to improve controllability in FIG. 9. Applying the power control voltage 33 in this construction allows the idle current value of the collector of the power amplifier transistor 2 to be reduced in the low output power.
However, when the operating current of the RF power amplifier transistor 2 is controlled (restricted) by controlling the Vctrl voltage 31 in the low output power, the power amplifier bias circuit shown in FIG. 9 needs to control the Vctrl voltage 31 in hundred mV units (e.g., controlling it to 2.8 V to 2.7 V), resulting in a problem that control is difficult and a special circuit or a high accuracy external regulator is required. Furthermore, the circuit shown in FIG. 9 operates with the diodes 14, 15 compensating for the temperature characteristic of the RF power amplifier transistor 2, but when the operating current of the RF power amplifier transistor 2 is controlled (restricted) by the Vctrl voltage 31 in the low output power, the current flowing through the diodes 14, 15 reduces, which results in a problem that the temperature compensation effect is reduced.
Though a proposal to solve the above described problem is presented in the example shown in FIG. 10, it requires the power supply 33 for bias control in addition to the Vctrl power supply 31 and the power supply 32 of the RF power amplifier transistor 2, which requires complicated control. Furthermore, its efficiency improvement effect is solely based on bias control.